The quantity of liquid product dispensed and the quality of the spray pattern are critical parameters which have a substantial impact on the performance of a liquid product applied via an atomized spray. This is particularly true when the liquid product is being utilized as a thin film coating on a surface (such as, for example, a cooking utensil or pan, a window, or even hair or skin), and the total quantity of liquid product applied and quality of the spray pattern directly impact the thickness and evenness of the product coating.
In view of the ever-increasing awareness and concern among consumers with respect to the use of chlorofluorocarbon (CFC) propellants (now largely discontinued due to their impact upon the ozone layer) and volatile organic compound (VOC) propellants (which aggravate low altitude pollution problems, and many are highly flammable), there has been a trend away from pre-pressurized hydrocarbon aerosol-type dispensing systems toward systems which utilize a manually-operated pump-type mechanism to force fluid through a specially-designed nozzle assembly to atomize the liquid product.
Many product formulations require the addition of thinning agents (such as water, alcohol, solvents, or other VOCs) in order to reduce the viscosity of the product to the point where it can be atomized with conventional, manually operated spray systems. However, such thinning agents are less than desirable from a consumer perspective because of their impact upon the performance of the product (such as the taste of food products), and (with some thinners such as alcohol or VOCs) the accompanying scent of the thinner and/or accompanying flammability problems. Other thinners such as water-based thinners may introduce microbial growth problems in the product.
While eliminating the use of thinners in product formulations addresses the problems encountered with such use, it is not without other problems. Comparatively higher viscosity (non-thinned) liquid products present an additional challenge in terms of atomization, as the liquid has a tendency to resist break-up rather than being dispensed as a finely dispersed mist. As a general proposition, the less finely dispersed the spray produced, the more difficult is it to achieve a comparatively thin and uniform layer of product on the intended surface, and hence product effectiveness in use is correspondingly diminished.
There are many products which may be applied to a surface via a manually operated spray system, including cleaning products, food products, surface coatings, and health and beauty care products. One particular product application of current interest is in the area of oil-based fluid products used in food preparation, such as pan coatings and flavor enhancers. A thin, even coating of the oil-based product is desirable in order to provide for non-stick baking characteristics in the pan coating context and to prevent over-application of flavor enhancers. Such products usually comprise a vegetable oil and may optionally include a small quantity of additives for stability, performance, and flavor enhancement. Other products of interest include hair sprays, which also require a thin even coating for satisfactory performance.
One currently commercially available pump sprayer for cooking oil products employs a nozzle design which produce two impinging jets of the product which collide outside the nozzle to atomize the liquid product. These impingement-type spray systems, particularly with comparatively higher viscosity product formulations, tend to produce a spray having a comparatively wide distribution of particle sizes. This is disadvantageous in terms of overall spray quality, as larger particles tend to travel farther than the smaller particles and tend to cause regions of heavier product concentration, while smaller particles tend to form a "cloud " of product which bounces off of the intended surface to be coated, only to float about in the air. By utilizing a finite number of impinging fluid streams, commercially available impingement-type systems also tend to produce a number of zones of heavier product application equal to or greater than the number of impinging streams. These tendencies generally become exaggerated with increasingly higher viscosities and smaller pump dosages.
The performance of these commercially available spray systems also suffers due to use of conventional pump technology which allows the product to emerge in a poorly atomized spray at the beginning and end of each pump stroke when the available pressure is less than required for atomization. Comparatively higher viscosity fluids typically have a narrower window of operating pressures which will provide satisfactory atomization than comparatively lower viscosity fluids, with such operating windows becoming increasingly narrow with increasing viscosity. Under some circumstances, such as when the pump is slowly actuated, a comparatively higher viscosity product fails to be atomized at all, and emerges from the nozzle assembly in a fluid stream. In this particular application, the result is wasted product and oversaturation of the food item or baking surface to be coated. Heavy drippage of product from the sprayer may also occur, which is generally messy and unsanitary in a food preparation environment.
While other commercially available dispensing systems employing single-orifice, swirl-type atomizing nozzles may work satisfactorily with comparatively lower viscosity formulations, their performance with comparatively higher viscosity formulations suffers due to several factors. First, viscous losses with comparatively higher viscosity fluids do not allow the fluid to attain enough swirl velocity to form a conical film, and thus do not facilitate break-up of the fluid into a finely dispersed spray. Second, the viscous nature of the fluid itself resists break-up of the fluid. The role of viscosity is to inhibit the growth of instabilities and generally delay the onset of disintegration. This causes atomization to occur farther downstream from the nozzle orifice in regions of lower relative velocity; consequently, drop sizes are comparatively larger. When the viscosity becomes too high, atomization is inhibited and streaming of the fluid occurs. Finally, while the first problem may be addressed if not overcome by the use of higher dispensing pressures, commercially available pump systems of this variety cannot sustain the pressures required to satisfactorily atomize comparatively higher viscosity fluids.
Thus, even though commercially available swirl-type atomization systems may in general (with comparable pressures and product viscosities) produce a narrower distribution of spray particle sizes and a more continuous (uniform) product distribution (without discrete zones of heavier application) than the commercially available impingement-type spray systems, their performance with comparatively higher viscosity formulations suffers because of their tendency to completely fail to atomize the product when the dispensing pressure is inadequate, resulting in a stream of liquid product being ejected from the nozzle.
Accordingly, it would be desirable to provide a manually operated pump-type product delivery system which would provide for a well-atomized, finely-dispersed spray of product with more uniform particle sizes and a more uniform spray pattern under all actuation circumstances when comparatively higher viscosity formulations are utilized.